Use 100% silicone lube like a grease or brake fluid. When you put them together the piston has to be perfectly aligned to get it started then you can c clamp it together. MAKE SURE YOU TAKE THE BLEADER OUT FIRST!
I had everything lubed up and was very careful to square up the piston. Still sheared the seal every time. Managed to get the first one in, oddly enough.
Then I applied a little compressed air, thinking the escaping air flow would force the seal out to let the piston in. Didn't help.